The present invention relates to a semiconductor film for use in thin film transistors (TFTs) used in liquid crystal displays, photosensors such as linear sensors, photovoltaic devices such as solar cells, memory LSIs for SRAMs (static random access memories) and the like. The invention also relates to a method and an apparatus for producing the semiconductor film. More particularly, the semiconductor film is a crystalline semiconductor thin film formed on, for example, a glass substrate or the like, by laser annealing an amorphous material or the like. The invention further relates to semiconductor device using the semiconductor thin film and a method of producing the device.
Conventionally, high-quality silicon semiconductor thin films used in thin film transistors (TFTs) or the like have been fabricated on an amorphous insulating substrates by using plasma CVD methods and plasma CVD apparatuses utilizing glow discharge. The hydrogenated amorphous silicon (a-Si) films produced by these manufacturing methods and apparatuses have been improved over many years of research and development and reached to a standard applicable as high quality semiconductor thin films. The hydrogenated amorphous thin films have found use in electric optical devices such as switching transistors for pixels in active matrix liquid crystal displays for lap-top or note-type personal computers, engineering workstations, and automobile navigation systems, photosensors for image sensors in facsimile machines, and solar batteries for electronic calculators, and in various integrated circuits. One of the most significant advantages of the hydrogenated amorphous silicon is that the formation with high reproducibility and stability on a large-area substrate is achieved at a process temperature as low as 300xc2x0 C.
Meanwhile, the recent advancement of increased device sizes and greater pixel densities (higher definitions) in displays and image sensors has led to the demand for silicon semiconductor thin films that can achieve further high speed driving. In addition, in order to reduce device weight and manufacturing cost, such thin films should be applicable to driver elements formed in the peripheral circuit area of a liquid crystal display, which also requires the thin films to be capable of operating at high speed. However, the foregoing amorphous silicon shows a field effect mobility of 1.0 cm2/Vxc2x7sec at best and thus cannot attain electric characteristics that can meet the requirements as mentioned above.
In view of the problems, research has been conducted on techniques for improving the field effect mobility and the like by forming a semiconductor thin film having crystallinity, and the developed manufacturing processes include:
(1) a manufacturing method in which by mixing a silane gas with hydrogen or SiF4, and employing a plasma CVD method, the deposited,thin film is crystallized; and
(2) a manufacturing method in which by using amorphous silicon as a precursor, the crystallization of the film is effected.
In the method (1), the crystallization proceeds along with the formation of the semiconductor thin film, and the substrate must be heated to a relatively high temperature (600xc2x0 C. or higher). This necessitates the use of a heat-resistant substrate such as costly quartz substrates, makes it difficult to use low-priced glass substrates, and therefore has a drawback of high manufacturing cost. Specifically, for example, Corning 7059 glass widely used in active matrix type liquid crystal displays has a glass transition temperature of 593xc2x0 C., and therefore if subjected to a heating treatment at 600xc2x0 C. or higher, the glass substrate will undergo considerable mechanical deformations such as shrinkage and strain, which makes it difficult to appropriately perform the forming processes of semiconductor circuits and the producing processes of liquid crystal panels. Furthermore, when a multi-dimensional integration is desired, there is a possibility of thermally damaging the previously-formed circuit area.
In the method (2) above, an amorphous silicon thin film is formed on a substrate, and the formed thin film is heated to obtain a polycrystalline silicon (polycrystal silicon: p-Si) thin film. This method generally utilizes a solid phase epitaxy method in which the heat treatment is performed at approximately 600xc2x0 C. for a long time, and a laser annealing method (especially an excimer laser annealing method).
In the solid phase epitaxy method, the substrate on which an amorphous thin film is formed needs to be heated and maintained at,a temperature of 600xc2x0 C. or higher for 20 hours or longer, and thus this method also has drawbacks of high manufacturing cost and so forth.
In the excimer laser annealing method, an amorphous silicon thin film is irradiated with an excimer laser light, which is a UV light having a large light energy, to cause crystallization, as disclosed in, for example, IEEE Electron Device Letters, 7 (1986) pp. 276-8, IEEE Transactions on Electron Devices, 42 (1995) pp. 251-7. This method thereby achieves, without directly heating the glass substrate, a polycrystalline silicon thin film having relatively good electrical characteristics such as a high field effect mobility (higher than 100 cm2/Vxc2x7sec). More specifically, amorphous silicon has a transmissivity characteristic as shown in FIG. 1 and, for example, shows an absorption coefficient of about 106 cmxe2x88x921 for a laser beam having a wavelength of 308 nm by a XeCl excimer laser. Therefore, most of the laser beam is absorbed in the layer from the surface to a depth of about 100 xc3x85, the substrate temperature is not raised significantly (to approximately not higher than 600xc2x0 C.), and amorphous silicon alone is brought to a high temperature to cause crystallization (polycrystallization or single crystallization). This allows the use of low-cost glass substrates. In addition, it is possible to irradiate a limited area of the substrate with the light beam to crystallize, and this allows a multi-dimensional integration in which a pixel region not requiring high speed characteristics so much is left to be an amorphous thin film, while a peripheral region of the pixel region is crystallized so as to form a driver circuit thereon. Further, it is also possible to form high quality crystalline thin films in a specific region on a substrate one after another, without thermally damaging the circuits already formed on the substrate. Furthermore, this technique permits the integration of CPUs (central processing units) and the like on the same substrate.
As an example of a semiconductor device using p-Si as described above, explained below is a typical construction of a TFT and a manufacturing method thereof.
FIGS. 2(a) and 2(b) schematically show a TFT 110 having a coplanar structure. FIG. 2(a) is a plan view of the TFT 110, and FIG. 2(b) is a cross sectional view thereof taken along the line P-Pxe2x80x2 in FIG. 2(a). As shown in FIGS. 2(a) and 2(b), the TFT 110 has an insulating substrate 111 on which there are provided a undercoat layer 112, a p-Si film 113, a first insulating film (gate insulting film) 114, a second insulating film 116, and three electrodes, namely, a gate electrode 115, a source electrode 117s, and a drain electrode 117d. The p-Si film 113 is a crystalline semiconductor layer composed of Si (silicon). The p-Si film 113 is formed on the undercoat layer 112, patterned in a predetermined shape. The p-Si film 113 comprises a channel region 113a, a source region 113b, and a drain region 113c, and the source region 113b and the drain region 113c are disposed on both sides of the channel region 113a. The source region 113b and drain region 113c are formed by doping impurity ions such as phosphorus ions and boron ions.
The first insulating film 114 is made of, for example, silicon dioxide (SiO2), and is formed over the p-Si film 113 and the undercoat layer 112. The gate electrode 115 is a metal thin film made of, for example, aluminum (Al) or the like. The gate electrode 115 is disposed above the first insulating film 114 at the position corresponding to the channel region 113a of the p-Si film 113. The second insulating film 116 is made of, for example, SiO2, and stacked above the gate electrode 115 and the first insulating film 114.
The first insulating film 114 and the second insulating film 116 each have a contact hole 118 formed so as to reach the source region 113b or the drain region 113c in the p-Si film 113. Via the contact hole 118, the source electrode 117s and the drain electrode 117d are in contact with the source region 113b or the drain region 113c. In a region not being the cross section shown in the figure, the gate electrode 115, the source electrode 117s, and the drain electrode 117d are patterned in a predetermined shape to form a wiring pattern.
The TFT 110 is produced in the following manner. First, the undercoat layer 112, made of for example SiO2, is formed on the insulating substrate 111. This prevents impurities from diffusing into a p-Si film 133 to be formed later. Next, on the undercoat layer 112, an a-Si film (not shown) as the foregoing amorphous silicon is deposited by, for example, a plasma CVD method. The a-Si film is then patterned into a predetermined shape by etching. It is noted that the patterning may be performed after the crystallization. Thereafter, the a-Si film is irradiated with an excimer laser having a short wavelength, and cooled (laser annealing). Thereby, the a-Si film is modified, i.e., the a-Si film is polycrystallized to form a p-Si film 113. Note here that the a-Si film has a large light absorption coefficient in a short wavelength range, and therefore, by employing an excimer laser as the energy beam, it is possible to selectively heat the a-Si film alone. Accordingly, temperature rise of the insulating substrate 111 is suppressed, and this leads to an advantage that low-cost substrates such as glass substrates can be employed for the insulating substrate 111.
On the p-Si film 113 thus formed, the first insulating film 114 is deposited by an atmospheric pressure CVD (chemical vapor deposition) method, and the gate electrode 115 is formed on the first insulating film 114. Thereafter, using the gate electrode 115 as a mask, impurity ions for serving as either donors or acceptors, specifically, such impurity ions as phosphorus ions and boron ions, are implanted into the p-Si film 113 by using, for example, an ion doping method. Thus, on the p-Si film 113, the channel region 113a, the source region 113b, and the drain region 113c are formed.
Next, the second insulating film 116 is formed on the gate electrode 115. Thereafter the contact holes 118 are formed, and aluminum, for example, is deposited, and patterning is performed to form the source electrode 117s and the drain electrode 117d. 
The pulsed type laser such as the excimer laser used in the forming process of p-Si films as described above has a large output power, and by irradiating the substrate with the laser light in such a manner that the substrate is scanned by the laser light with a line-like shape while the substrate is being moved, a large-area amorphous silicon can be crystallized at one time, which is an advantage in the large-scale production of semiconductor devices. However, such a laser has a drawback that it is difficult to improve quality of the formed crystals. Specifically, a laser of this type has a very short irradiation time for 1 pulse; namely approximately several ten nanoseconds, and causes a large temperature difference between a time during which the irradiation is performed and a time during which the irradiation is not performed is large, and thereby, the fused silicon film crystallizes during the process of being rapidly cooled. Accordingly, the control of a degree of crystal growth and a crystal orientation is difficult, which leads to many drawbacks as follows: sufficient crystal growth tends not to occur, thereby crystal grain sizes are reduced and densities of grain boundaries are increased, uneveness in crystals is increased, and further crystal defects tend to increase. More specifically, in the process of cooling after the irradiation with the laser, crystal nuclei are formed in a disorderly manner, and the disorderly-formed crystal nuclei in turn grow in disorderly directions. The crystal growth stops in a state where crystal grains collide with each other. The crystal grains formed through such a growth process results in small grains having random shapes. Consequently, in the resulting poly-Si film, a large number of grain boundaries are present and therefore charge carriers cannot transfer smoothly, which causes the film to have poor TFT characteristics such as field effect mobilities.
Now, the mechanism of the crystal growth and the reasons why a good crystal growth is difficult to attain are detailed below. The aforesaid excimer laser is generated by exciting a mixed gas containing a rare gas such as Xe and Kr and a halogen such as Cl and F with the use of an electron beam. However, the laser as it is cannot be suitably used. By using an optical system called a beam homogenizer, the light beam generally used in laser annealing is shaped into a light beam having a homogeneous light intensity and having a line-like shape or a rectangular shape with each line segment being about several centimeters. The light beam thus shaped is employed for the technique in the crystallization of non-single crystalline thin films (normally, amorphous thin films), the technique in which the substrate is scanned with the light beam.
Yet, this technique has several problems to be solved, such as poor uniformity in crystal grain sizes and in crystallinity, unstable transistor characteristics, and low field effect mobilities. In view of these problems, the following techniques have been suggested.
(1) By covering part of the surface to be irradiated with a reflective film or absorption film so as to control light absorption on the surface of the thin film, a light intensity distribution is formed to guide an orientation of crystal growth. (2) A substrate is heated (at 400xc2x0 C.), and the laser is applied to the heated substrate, so that a smooth crystallization takes place. (Extended Abstracts of the 1991 International Conference on Solid State Devices and Materials, Yokohama, 1991, pp. 623-5)
Additionally, a technique disclosed in Jpn. J. Appl. Phys. 31 (1992) pp. 4550-4 is also known.
As shown in FIG. 3, a glass substrate 121 having a precursor semiconductor thin film 122 formed thereon is placed on a substrate stage 124. With a substrate stage 124 heated at about 400xc2x0 C. by a substrate heater 125, a laser beam 123a of an excimer laser 123 is applied to the precursor semiconductor thin film 122. The additional heating of the glass substrate in the irradiation with laser beam achieves high crystal quality, i.e., relatively large and uniform crystal grains, thereby improving the electrical characteristics.
Technique (1) described above can be employed to obtain a single crystal, whereas technique (2) above is relatively easy to implement and can suppress the variation of field effect mobilities within xc2x110%. However, these techniques have the following problems and cannot meet the requirements in recent technical trends towards multi-dimensional integration and further cost reduction.
Specifically, technique (1) above requires a step of providing a reflective film etc., which complicates the manufacturing process and adds the manufacturing cost. Furthermore, since providing a reflective film etc. in a narrow, limited space is difficult, crystallization of specific regions with a very small size is accordingly problematic.
On the other hand, technique (2) involves a step of heating the substrate, which reduces the productivity. Although the substrate is not heated at such a temperature as required in a solid phase epitaxy method, the step of heating and cooling the substrate takes a long time (30 minutes to 1 hour, for example), which reduces the throughput. This problem becomes more serious as a substrate area increases, since a larger substrate requires a longer time for heating and cooling to alleviate the deformation of the substrate. In addition, this technique can reduce the variation of field effect mobilities to a certain extent, but cannot sufficiently increase the field effect mobilities per se, and therefore it is not suitable for producing circuits in which high speed operation is required. In order not to cause the deformation etc. of the glass substrate, the substrate cannot be heated in excess of 550xc2x0 C., and therefore it is difficult to attain further higher crystal quality. Moreover, this technique involves heating the entire substrate, and therefore is not suitable for the crystallization in a limited region (specific region) on the substrate.
As has been described above, both techniques (1) and (2) above have problems such as high manufacturing cost. Particularly, techniques (1) and (2) above (including other prior art techniques) have a significant problem of the difficulty in realizing diverse and multi-dimensional layer stacking. In other words, the methods for controlling the temperature distribution employed by these techniques are not suitable for selectively forming on a single substrate both a circuit region where high speed operation is required (polycrystallized region) and another circuit region where not such high speed is required (amorphous region). For this reason, by these techniques, it is difficult to achieve both a high degree of integration and cost reduction at the same time.
The technique capable of crystallizing only a predetermined limited region is useful, and this will be elaborated upon now. Prior art laser annealing methods have employed a light beam having such a characteristic that the side (edge) of the beam is steep, and the top is flat (the energy intensity per unit area is uniform), as shown in FIG. 4. Poly-Si thin films produced with the use of such a light beam have been conventionally seen as sufficient for the requirements for forming switching circuits and the like for pixel electrodes, for which such high speed operation is not required.
However, in the case of integrally forming the elements requiring high speed operation such as CPUs as well as a gate driving circuit and a source driving circuit on a single substrate, the polycrystalline thin films of the quality realized by the prior art technique are unsatisfactory. For example, the pixel region of LCDs generally requires a mobility of about 0.5-10 cm2/Vs, whereas the peripheral driving circuits for controlling the pixels, such as gate circuits and source circuits, require a mobility as high as about 100-300 cm2/Vs. In spite of such requirements, the prior art technique utilizing the light beam having the above-described characteristic cannot achieve high mobilities constantly. In other words, generally in polycrystalline silicon thin films, higher transistor characteristics are attained as the crystal grain size is increased, but the above-described polycrystallization treatment cannot attain sufficient transistor characteristics.
The reason is that when the light beam having the above-described characteristic is employed, uneveness in crystal grains and crystallinity is increased, and in addition, when the irradiation intensity and the number of times of the irradiation are increased to improve the crystallization, the crystal grain sizes become more uneven, further varying the crystallinity. The cause of this problem will now be discussed in detail below.
FIG. 5 schematically shows the distribution of crystallinity in the case where the above-described rectangular light beam is applied to an amorphous silicon thin film formed on a substrate. In FIG. 5, the numeral 1701 refers to the boundary of the irradiation beam, the numerals 1702 and 1704 refer to the portions where the crystallinity is low, and the shaded portion 1703 indicates the portion where the crystallinity is high. As shown in FIG. 5, by the prior art method utilizing an excimer laser having a uniform energy intensity, the crystallinity shows such a distribution pattern that only the shaded portion 1703 slightly inside the boundary of the irradiation light shows high crystallinity, whereas the crystallinity is low in the other portions (the adjacent portion 1702 to the boundary, and the central portion 1704) show low crystallinity. This has been confirmed by micro-Raman spectroscopy.
FIG. 6 shows the measurement result of Raman intensity on the portion along the line Axe2x80x94A in FIG. 5. The abrupt peaks in FIG. 6 existing slightly inside the boundary indicate that the crystallinities in these portions are high. In addition, the absence of peaks in the central portion shows that the crystallinity in the central portion is low.
Referring now to FIG. 7, the mechanism of such uneveness in crystallinity will be discussed below. An amorphous silicon thin film is irradiated with a light beam to heat the thin film so that the temperature of the thin film is increased above the melting point temperature of silicon (about 1400xc2x0 C. or higher), and thereafter the light irradiation is stopped. Thereby, the temperature of the thin film decreases by heat dissipation, and in this process, the fused silicon crystallizes. Here, when the light beam has a uniform distribution of the light intensity as in FIG. 7(a), the surface of the irradiated thin film shows a temperature distribution pattern as in FIG. 7(b). That is, a flat temperature region in which no temperature gradient is present is formed in the central portion, and an abrupt temperature gradient is formed in the peripheral regions since the heat escapes outside. In this case, when the temperature of the central portion is higher than the melting point of silicon, the temperature in the portions adjacent to the intersection points of the temperature distribution curve 1901 and the crystallization temperature line 1902 (adjacent portions to the boundary) reaches the crystallization temperature first after the irradiation is stopped. Therefore, crystal nuclei 1903 are formed in the vicinity of these portions (see FIG. 7(c)). In other words, the temperature of the amorphous silicon film is increased at a temperature higher than the melting point, and when the amorphous silicon thin film is fused and then solidified in the regions heated above the melting point, crystallization occurs, resulting in polycrystatlization. Following this, as the temperature further decreases (FIG. 7(d)), the crystallization further proceeds from the crystal nuclei 1903 as the starting points towards the central portion, where the temperature has not yet reach the crystallization temperature (FIG. 7(e)). In the case of using the light beam having a uniform energy intensity, the temperature decreases in such a manner that no temperature gradient in the surface direction in the central portion is present, as shown in FIGS. 7(b), 7(d), and 7(f). Accordingly, at a certain point during the temperature decrease, a relatively wide region reaches the crystallization temperature at one time (FIG. 7(f)), and the crystal nuclei can be formed at any point of the wide region 1904 with an equal probability. Therefore, as shown in FIG. 7(g), micro crystal nuclei are simultaneously formed on the entire surface of the region 1904, and as a result, a poly-Si thin film made of a multiplicity of microcrystal grains is formed. Such a poly-Si thin film inevitably has a large density of grain boundaries. Therefore, the degree of carriers being caught in the grain boundaries increases, reducing the field effect mobility. It is noted that in FIG. 7(c), the numeral 1900 represents a cross section of the thin film.
The above-described mechanism for the uneveness of crystallinity being caused also applies to the case of employing a line-like laser beam for the irradiation, as shown in FIG. 8. FIG. 8(a) illustrates the distribution of energy densities in the directions x and y of an excimer laser to be used. FIG. 8(b) shows the distribution of the increased temperatures of the amorphous silicon thin film irradiated with the excimer laser having such energy densities. FIG. 8(c) is a perspective view of a polycrystalline silicon thin film transistor irradiated with the laser as shown in FIGS. 8(a) and 8(b). As seen from these figures, since the laser having such an energy distribution as shown in FIG. 8(a) is employed, the region to be irradiated shows a temperature distribution almost uniform along the y direction, but shows such a temperature distribution in the x direction that the central portion is high and both side portions are low, as shown in FIG. 8(b). Due to such a temperature distribution, the crystallization proceeds from the peripheral regions towards the central region along the x direction, and the crystallization growth fronts of the numerous formed crystal nuclei from both peripheral regions meet at the central region. Therefore, as shown in FIG. 8(c) which schematically illustrates the state of the crystallization in the polycrystalline silicon thin film, although the crystal grain size is large in the regions where the line beam energy density of the laser beam is low, the grain size becomes small in the region where the energy density is high (the central region). For reference, in FIG. 8 (c), the numeral 131 designates the transparent insulating substrate, the numeral 134 the polycrystal silicone thin film, and the numeral 141 the crystal grains. The numeral 139 refers to an insulating film which generally composed of a silicon dioxide (SiO2) film, and the numeral 140 represents the amorphous silicon thin film.
Although, for the sake of brevity, the above-described example describes a case where the energy beam is applied only one time, the same explanation applies to such cases that the energy beam is applied a plurality of times.
In addition to the difficulty in improving field effect mobilities, prior art laser annealing methods have other drawbacks of the difficulty in improving the uniformity in quality of semiconductor films and of the difficulty in meeting both these requirements.
Referring now to FIG. 9, a prior art leaser annealing apparatus is described below. In FIG. 9, the numeral 151 designates a laser oscillator, the numeral 152 a reflector, the numeral 153 a homogenizer, the numeral 154 a window, the numeral 155 a substrate having an amorphous silicon layer formed thereon, the numeral 156 a stage, and the numeral 157 a control unit. The laser annealing apparatus is so constructed that a laser light emitted from the laser oscillator 151 is guided to the homogenizer 153 by the reflector 152, and the laser beam shaped by the homogenizer 153 in a predetermined shape with a uniform energy is applied through the window 154 onto the substrate 155 fixed on the stage 156 in the treatment chamber.
When performing an annealing treatment with the use of the above-described annealing apparatus, because it is difficult to irradiate the entire substrate surface with a laser beam at one time, the regions to be irradiated is in turn staggered so that the region to be irradiated overlaps with the already irradiated region, in order to irradiate the entire substrate surface under the same condition. See for example, I. Asai, N. Kato, M. Fuse, and T. Hamano, Jpn. J. Apl. Phys. 32 (1993) p.474. However, in such a laser annealing method, in which the laser beam is applied in such a manner that the regions to be irradiated are in turn staggered so that the region to be irradiated overlaps with the already irradiated region, increasing the laser energy density can lead to an increase in the mobility, which is one of the evaluation standards for semiconductor film characteristics, thereby increasing the film quality as a whole. However, the increased laser energy density also increases non-uniformity of the film quality at the overlapped regions, thus degrading the uniformity of the semiconductor film as a whole. On the other hand, when a relatively low energy density is employed in the laser irradiation, improving the uniformity of the film quality becomes easier, but because of the low energy density, increasing the field effect mobility becomes difficult.
Accordingly, in the case of employing a substrate having TFTs formed thereon for a liquid crystal display as schematically shown in FIG. 10, it has been difficult to form a semiconductor film which satisfies both uniformity of the film quality, required for an image display region 158 having a relatively large area, and field effect mobilities, required for a peripheral circuit (driver circuit) region 159. It is to be noted here that as a solution to this problem, U.S. Pat. No. 5,756,364 suggests that the image display region 158 and the peripheral circuit region 159 be irradiated with laser beams with different intensities. However, by merely varying the laser beam intensities, it is still difficult to obtain a field effect mobility sufficient for the peripheral circuit region 159.
Hence, as has been described above, prior art laser annealing methods have the following drawbacks. Controlling crystal grain sizes and crystal orientations is difficult and thereby forming semiconductor thin films having high crystal quality, i.e., large and uniform crystal grain sizes and few crystal defects. In addition, the reduction in manufacturing cost by increasing throughputs is difficult. Furthermore, prior art methods cannot achieve the improvement in film characteristics of semiconductor thin films (field effect mobilities etc.) and the uniformity of the film quality at the same time.
In view of the foregoing and other problems of prior art, it is an object of the present invention to provide a method of producing a semiconductor thin film that can achieve a semiconductor thin film having a high crystal quality without sacrificing throughputs, and can also achieve both an improvement in film characteristics and a uniformity in film quality in the semiconductor thin film at the same time.
It is another object of the present invention to provide an apparatus for producing such a semiconductor thin film.
It is further another object of the present invention to provide a thin film transistor having excellent TFT characteristics such as field effect mobilities and the like, by utilizing the semiconductor thin film.
It is yet another object of the present invention to provide a method for producing such a thin film transistor.
It is to be noted here that the term xe2x80x9ccrystallizationxe2x80x9d as used herein means both single crystallization and polycrystallization, and that a method for producing a crystalline semiconductor thin film according to the present invention is particularly useful in producing poly-Si thin films.
In view of the foregoing and other problems of prior art, it is an object of the present invention to provide a method of producing a semiconductor thin film that can achieve a semiconductor thin film having a high crystal quality without causing the reduction in throughputs, and can also achieve both an improvement in film characteristics and a uniformity in film quality in the semiconductor thin film at the same time.
It is another object of the present invention to provide an apparatus for producing such a semiconductor thin film.
It is further another object of the present invention to provide a thin film transistor having excellent TFT characteristics such as field effect mobilities and the like, by utilizing the semiconductor thin film.
It is yet another object of the present invention to provide a method for producing such a thin film transistor.
In order to provide a solution to the foregoing problems, the present inventors have, as a result of intensive studies, found a method of forming at least a region in which a transistor is formed into a polycrystalline silicon thin film having a large grain size, based on the consideration that a cause of crystal grains in a polycrystalline silicon thin film being undesirably small is a temperature variation in the silicon thin film heated by irradiating with an excimer laser.
More specifically, the present inventors have reached the following idea; when performing a polycrystallization treatment by laser, by providing a region having a high thermal conductivity on both sides of the region in which a transistor is formed so as to sandwich the transistor-forming region, the temperature of the peripheral regions sandwiching the transistor-forming region is made higher than the temperature of the transistor-forming region, which makes the temperature of the transistor forming region lower relative to the peripheral regions, and thereby the silicon film in the transistor-forming region is initially crystallized to increase crystal grain sizes.
Accordingly, in accordance with a first aspect of the invention, there is provided a method of producing a semiconductor thin film comprising the steps of stacking on a substrate a first insulating film having a first thermal to conductivity and a second insulating film having a second thermal conductivity being different from the first thermal conductivity, the second insulating film selectively formed in a partial region on the substrate, stacking a non-single crystal semiconductor thin film over the first insulating film and the second insulating film, and irradiating the non-single crystal semiconductor thin film with an energy beam to effect a crystal growth.
Specifically, for example, in an insulating film under an amorphous silicon thin film, a thermal conductivity of a region in which a transistor is formed is made different from a thermal conductivity of other regions, and thereby a thermal conductive performance of the amorphous silicon thin film in the region in which a transistor is formed is made higher than that of the amorphous silicon thin film in the other regions.
By employing this method, in the polycrystallization, the temperature of the silicon thin film in the region where a transistor is formed is lower than that of the other regions, and as a result, crystallization starts from the region in which a transistor is formed. Consequently, a grain size of the polycrystalline silicon in the region in which a transistor is formed can be made large.
In accordance with another aspect of the invention, in at least a part of a peripheral edge of the semiconductor thin film, at least one protruding part extending towards a horizontal direction with respect to the semiconductor film may be provided.
Now, for a better understanding of the present invention, an approach which the inventors have taken to reach the present invention is described below. First, the present inventors made strenuous efforts in order to discover the cause of the foregoing problems in prior art, and reached the following factors as the cause. Generally, the generation of crystal nuclei and the crystal growth are effected by heating a semiconductor film by an annealing treatment and thereafter cooling the semiconductor film. In the prior art, the semiconductor film is almost uniformly cooled after the annealing treatment regardless of whether it is in the central region or in the peripheral region, and as a result, crystal nuclei start to develop at random positions almost at one time. It is considered that this makes it difficult to control crystal grain sizes and crystal orientations. Also for the same reason, crystal nuclei start to develop at relatively adjacent positions almost at one time and thereby the crystals tend to interfere with each other in the process of crystal growth. This makes it difficult to obtain a sufficient crystal grain size.
Based on the above factors, the present inventors have made strenuous studies and reached a technical idea of the present invention that xe2x80x9cthe formation of crystal nuclei in the peripheral region in the semiconductor film is started earlier than the formation of crystal nuclei in the central region, and thereafter the crystal nuclei formed in the peripheral region are grown towards the central region before crystal nuclei start to form or grow, in order to control crystal grain sizes and crystal orientations and to obtain a sufficient crystal grain size by preventing the crystals undergoing crystal growth from interfering with each other.xe2x80x9d
More specifically, in accordance with this aspect of the invention, in the semiconductor film after the annealing treatment, the heat accumulated in the protruding part in the peripheral edge diffuses in a plurality of outward directions with regard to a horizontal plane (for example, in three directions in the case of the protruding part having a rectangular shape), whereas the heat accumulated in the central region can escape, with regard to a horizontal plane, only in the directions towards the peripheral edge, which has not yet been cooled, and therefore the peripheral edge region, including the protruding part, can be cooled sufficiently earlier than the central region.
Accordingly, the crystal nuclei in the peripheral edge region start to form earlier than those in the central region, and the crystal nuclei in the peripheral region grow towards the central region before crystal nuclei are formed or grown in the central region, which makes it possible to control crystal grain sizes and crystal orientations. Thereby, it is prevented that the crystals in the process of crystal growth interfere with each other, and a sufficient crystal grain size can be obtained.
In another embodiment of this aspect of the invention, the protruding part may have a size such that one crystal nucleus is formed when crystal growth takes place by irradiating with the energy beam. Accordingly, only one crystal nucleus is formed in the protruding part, and the crystal nucleus is grown to be a crystal. In still another embodiment of this aspect of the invention, a length of said protruding part in a direction of protruding is greater than a film thickness of said semiconductor thin film, and equal to or less than 3 xcexcm, or a width of the protruding part in a width direction perpendicular to the direction of protruding is greater than the film thickness of the semiconductor thin film, and equal to or less than 3 xcexcm. Accordingly, the crystal grain size is further finely adjusted, and it is ensured that one crystal nucleus is formed in each protruding part.
In another embodiment of this aspect of the invention, the semiconductor thin film may be formed in a shape having a pair of line segments opposed to each other; two or more of the protruding parts may be formed in each of the pair of line segments; and an interval of the protruding parts next to each other in each of the pair of line segments may be set to be approximately equal to an interval of the opposing line segments.
Accordingly, the crystal nucleus formed and grown in the protruding part further grows towards the central region, and it is expected that crystal growth is effected such that both the crystals grown from the protruding parts next to each other towards the central region and the crystals grown from the protruding parts in the opposed line segments towards the central region are grown with a minimum interference with each other.
In another embodiment of this aspect of the invention, there is provided a semiconductor device comprising a semiconductor thin film wherein crystals are grown by irradiating a non-single crystal semiconductor thin film with an energy beam, characterized in that a protruding part is formed in a peripheral edge region of the semiconductor thin film, the protruding part extending outwardly in the same plane as a plane of the semiconductor thin film.
In the above device, a thin film transistor having a source region, a gate region, and a drain region each made of the semiconductor thin film may be formed, and the protruding part may be formed at least in a peripheral edge region of the gate region.
Accordingly, the protruding part is provided in the region corresponding to the gate electrode, and thereby a good electrical conductivity is obtained.
In another embodiment of the invention, there is provided a method of producing a semiconductor thin film, comprising the steps of forming a non-single crystal semiconductor thin film having a protruding part extending outwardly in the same plane as a plane of the non-single crystal semiconductor thin film, and growing crystals in the non-single crystal semiconductor thin film by irradiating with an energy beam.
In a semiconductor film produced according to the above-described method, the same advantageous effects as described above are attained.
In another embodiment of the invention, crystal nuclei in a peripheral region in the non-single crystal semiconductor thin film are formed earlier than crystal nuclei in a central region in the non-single crystal semiconductor thin film, and thereafter, the crystal nuclei in the peripheral region are grown towards the central region before the crystal nuclei in the central region start to be formed or grown. Thereby, it is made possible to control crystal grain sizes and crystal orientations.
This achieves a sufficient crystal grain size since it is prevented that crystals in the process of crystal growth interfere with each other.
In order to the foregoing and other problems in prior art, the present invention also provides a crystalline thin film transistor including a crystalline semiconductor layer formed on a substrate, the crystalline semiconductor layer comprising a channel region, a source region disposed at both sides of the channel region, and a drain region, wherein the crystalline semiconductor layer is such that a non-single crystalline thin film is crystallized, and at least in the channel region in the crystalline semiconductor layer, a gap for controlling an orientation of crystal growth is provided.
In the above-described configuration, a gap for controlling an orientation of crystal growth formed in the channel region controls an orientation of crystal growth in the channel region when the non-single crystalline thin film is crystallized. Accordingly, in the crystalline semiconductor layer having such a gap for controlling an orientation of crystal growth, shapes and density of grain boundaries of the crystals are preferably controlled, and hence the crystalline thin film transistor exhibits excellent TFT characteristics, such as field-effect mobility.
It is to be noted here that the gaps for controlling an orientation of crystal growth is a depressed part formed on the surface of crystalline semiconductor layer (non-single crystalline thin film in the process of fabrication), and the depressed part may extend to a lower layer under the crystalline semiconductor layer (the surface of the substrate or the undercoat layer) or may not extend to the lower layer. The sizes and shapes thereof are not particularly restricted, and therefore may be suitably adjusted depending on the surface area and thickness of the crystalline semiconductor layer, a desired field effect mobility, and so forth. Examples of the surface shapes thereof include a circular shape, a square-shaped hole, a long and narrow groove, and so forth, and examples of the cross-sectional shape thereof include a C-like shape, a V-like shape, and an angular C-like shape. The detail of the gap for controlling an orientation of crystal growth will be described later.
In another embodiment of the invention, there is provided a crystalline thin film transistor including a crystalline semiconductor layer formed on a substrate, the crystalline semiconductor layer comprising a channel region, a source region disposed at both sides of the channel region, and a drain region, wherein the crystalline semiconductor layer is such that a non-single crystalline thin film is crystallized, wherein the gap for controlling an orientation of crystal growth is divided into two or more arrays of groove-like gaps provided in a direction linking the source region and the drain region.
In this configuration, two or more arrays of groove-like gaps functions so that the orientation of crystal growth is guided in the direction linking the source region and the drain region, and the resulting poly-Si film becomes an aggregate of large crystal grains longitudinally extending in the direction linking the source region and the drain region. Such a poly-Si film has a small density of grain boundaries in the direction linking the source region and the drain region and therefore exhibits a high carrier mobility. In other words, the crystalline thin film transistor having the above-described configuration has excellent characteristics such as carrier mobilities.
Now, referring to FIGS. 11 and 20, there is detailed the reason why by providing the gap for controlling an orientation of crystal growth, large crystal grains in which the orientation of crystal growth is controlled are obtained.
As shown in FIGS. 20(a) and 20(b), on a surface of non-single crystalline thin film, which is a precursor material of the crystalline semiconductor layer, two or more arrays of groove-like gaps for controlling an orientation of crystal growth (denoted by the reference numeral 411) are formed in the direction linking the source region and the drain region, and thereafter an energy beam capable of being absorbed is applied to the thin film in a conventional manner. The resulting temperature distribution on the surface of the thin film is as follows; the gaps for controlling, the region adjacent thereto, and the peripheral edge region show a low temperature, and the main portion of the channel region (a region of the thin film where the gap for controlling an orientation of crystal growth is not formed) shows a high temperature.
The reason is that, since the groove region (gaps for controlling an orientation of crystal growth) has a smaller film thickness than the other regions or has no thin film thereon, less energy beam is absorbed in the groove region, and as a result the temperature of the groove region becomes lower than the other regions. In addition, normally, because no thin film exists outside the semiconductor thin film, which leads to less absorbed energy beam, and also because heat diffuses outward in the peripheral edge region, the temperature of the peripheral edge region becomes lower than the central region of the thin film.
There is now described below a process of crystal growth in the non-single crystalline thin film having a temperature distribution such that the gap for controlling an orientation of crystal growth and the peripheral edge region show a low temperature. It is noted that in prior art as well, the peripheral edge region of non-single crystalline thin film shows a low temperature, and therefore the explanation here concerns with the relationship between orientations of crystal growth and the gap for controlling an orientation of crystal growth, with reference to FIGS. 11(a) and 11(b).
FIGS. 11(a) and 11(b) schematically illustrate a state of the crystal growth. First, crystal nuclei are formed in a peripheral region of the gap for controlling an orientation of crystal growth, where the temperature is lower than that of the main portion. Then, the crystal nuclei grow in a direction towards a region having a higher temperature, i.e., in a direction away from the groove-like gap for controlling an orientation of crystal growth (a perpendicular direction to the groove), as the temperature of the whole thin film falls. Note here that in the above-described configuration, two or more array of the gaps for controlling an orientation of crystal growth are provided in the direction linking the source region and the drain region, and therefore, the crystal nuclei formed in the regions adjacent to the two gaps an orientation of crystal growth opposed to each other grow towards the center of the main portion of the channel region from the opposite directions. Therefore, both crystal grains collide with each other in the vicinity of the center of the main portion of the channel region. However, the central region, being far from the gap for controlling an orientation of crystal growth, has still a higher temperature than the other regions and therefore is in a state where molecules therein can still freely move. Accordingly, the orientation of crystal growth is guided to the direction in which the collision is avoided, i.e., the direction linking the source region and the drain region (the direction parallel to the groove, see FIG. 11a). As a result, a large crystal grain is formed so as to longitudinally extend in the direction linking the source region and the drain region (see FIG. 11b ). When the channel region is composed of an aggregate of the crystal grains having such a shape, the density of grain boundaries in the direction linking the source region and the drain region becomes small, and therefore a crystalline thin film transistor having excellent TFT characteristics such as field effect mobility can be formed.
In another embodiment of the invention, the gap for controlling an orientation of crystal growth may be divided into a plurality of gaps discontinuously provided in a direction linking the source region and the drain region.
When a plurality of gaps for controlling an orientation of crystal growth are discontinuously arranged, crystal growth is more finely controlled, and in particular, when two or more arrays of gaps for controlling. an orientation of crystal growth are arranged, grain sizes and shapes of the crystals are further finely controlled. The reasons for this are as follows.
As described above, crystal nuclei are formed in the vicinity of the gap for controlling an orientation of crystal growth, in which the temperature decreases to a crystallization temperature earlier. Here, if the intervals between the crystal nuclei are narrow, the crystal growth is hindered since crystals collide with other crystals before they sufficiently grow, and this results in a polycrystal made of a multiplicity of micro-crystal grains and a distorted crystal structure in the vicinity of the boundaries where crystal grains collide with each other. For this reason, desired TFT characteristics cannot be obtained. Accordingly, in order to improve TFT characteristics such as field effect mobility, it is necessary that the density of crystal nuclei to be formed should be appropriately controlled, in addition to the control of the orientation of crystal growth.
Here, if the gaps are arranged in a discontinuous manner, although crystal nuclei are formed in the vicinity of the gaps, they are not easily formed in intermediate regions between a gap and the next gap. Thus, by adjusting the number of the gaps and/or the intervals between the gaps, the density of crystal nuclei to be formed can be controlled. It is noted that the reason why crystal nuclei are not easily formed in the intermediate region between a gap and the next gap is that the intermediate region (the region where the thin film material is present) is sufficiently heated by laser irradiation.
In accordance with another aspect of the invention, there is provided a, semiconductor device including a crystalline semiconductor layer formed on a substrate, the crystalline semiconductor layer comprising a channel region, a source region disposed at both sides of the channel region, and a drain region, the semiconductor device wherein the crystalline semiconductor layer is such that a non-single crystalline thin film is crystallized and at least in the channel region, an early-crystallization region in which a crystallization-starting temperature is higher than that in a main portion of the channel region is provided.
By employing the above-described configuration, the early-crystallization region serves to control the crystal growth in the main portion of the channel region, and as a result, high quality crystalline semiconductor layer having a small density of grain boundaries can be formed. The reason is as follows.
Because the crystallization-starting temperature is high in the early-crystallization region, crystal nuclei start to be formed earliest in the early-crystallization region. The crystal nuclei become the center of the crystal growth to take place thereafter. Accordingly, by providing the early-crystallization region, the phenomenon that multiple crystal nuclei are formed at once can be prevented, and as a result, a polycrystalline semiconductor layer in which large crystal grains are aggregated can be formed.
It is preferable that the early-crystallization region be arranged at least one or more in the channel region, and it is also preferable that a plurality of early-crystallization regions be provided at positions where the transfer of carriers in the direction linking is not hindered. When a plurality of early-crystallization regions are provided on the surface of the thin film at appropriate positions and intervals, the density of crystal nuclei to be formed can be appropriately controlled, resulting in further desirable results. It is to be noted that the phrase xe2x80x9ca crystallization-starting temperature is highxe2x80x9d mentioned above means that crystallization starts to take place at a higher temperature than that in the main portion of the channel region.
In another embodiment of the invention, the early-crystallization region has a shape longitudinally extending in a direction linking the source region and the drain region.
Since the early-crystallization region is not a region where carrier transfer takes place, the region is preferable to have a narrow width in the direction linking the source region and the drain region. If the early-crystallization region extends longitudinally in the direction linking the source region and the drain region, the early-crystallization region can become a factor that hinders carrier mobilities.
In another embodiment of the invention, the early-crystallization region is such that an impurity is contained in a component constituting the main portion of the channel region.
In a technique of raising the crystallization-starting temperature by adding an impurity to the semiconductor layer, the early-crystallization region can be formed relatively easily. Accordingly, the crystalline thin film transistor having the above-described configuration not only exhibits excellent TFT characteristics such as field effect mobility but also achieves a reduced cost.
In another embodiment of the invention, the crystalline semiconductor layer is substantially composed of silicon or a compound of silicon and germanium.
Silicon and a compound of silicon and germanium are readily available and easy to crystallize. Accordingly, the above-described configuration achieves a high quality crystalline thin film transistor at a low cost.
The methods of producing a semiconductor device which will be described below relate to the crystalline thin film transistors which has been described above, and the advantageous effects by the methods are almost the same as those described above. For this reason, the detailed explanation for the advantageous effects will not be repeated below.
In accordance with another aspect of the invention, there is provided a method of producing a crystalline thin film transistor including a crystalline semiconductor layer, the crystalline semiconductor layer comprising a channel region, a source region disposed at both sides of the channel region, and a drain region, the method comprising at least the steps of depositing a non-single crystalline thin film on an insulating substrate, forming a plurality of gaps for controlling an orientation of crystal growth in the non-single crystalline thin film, and irradiating the thin film in which the plurality of gaps are formed with an energy beam to crystallize the thin film.
In the above-described method, the gaps for controlling an orientation of crystal growth may be formed in a direction linking the source region and the drain region so as to have a groove-like shape, and in addition, the gap for controlling an orientation of crystal growth may be divided into a plurality of gaps discontinuously formed in a direction linking the source region and the drain region. By employing these methods, the foregoing crystalline thin film transistors can be produced.
In accordance with another aspect of the invention, there is provided a method of producing a crystalline thin film transistor including a crystalline semiconductor layer, the crystalline semiconductor layer comprising a channel region, a source region disposed at both sides of the channel region, and a drain region, the method comprising at least the steps of depositing a non-single crystalline thin film on an insulating substrate, forming an early-crystallization region by ion-implanting an impurity in a partial region in the non-single crystalline semiconductor thin film, the impurity for raising a crystallization-starting temperature of the partial region, and after the step of forming an early-crystallization region, irradiating the thin film with an energy beam to crystallize the thin film.
In the above-described method, the early-crystallization region may have a belt-like shape longitudinally extending in a direction linking the source region and the drain region, and in addition, the early-crystallization region is divided into a plurality of early-crystallization regions discontinuously disposed in a direction linking the source region and the drain region. By employing these methods, the foregoing crystalline thin film transistors can be produced.
In addition, in each of the foregoing producing methods, the energy beam may be an excimer laser beam.
Excimer lasers have a large light energy, and is well absorbed by silicon since they are UV lights. Therefore, by using an excimer laser beam, crystallization of the non-single crystalline semiconductor layer can be efficiently performed. In particular, when the non-single crystalline semiconductor layer is composed of a material capable of absorbing ultraviolet rays such as silicon, it is possible to selectively heat and fuse only the semiconductor layer. Therefore, the crystallization of the semiconductor layer can be effected without causing adverse effects by heat on the regions not irradiated with the beam, and moreover, it is made possible to employ glass substrates, which are low in cost. Furthermore, when an excimer laser and a thin film material capable of absorbing UV are used in combination, the temperature difference between the gap for controlling an orientation of crystal growth and the main portion of the semiconductor layer becomes large, and thereby the function of the gap for controlling an orientation of crystal growth (function for controlling the orientation of crystal growth) can be fully exploited.
The present inventors have also carried out a study on methods for sufficiently growing crystals based on the foregoing considerations regarding the mechanism of crystallization. As a consequence, the inventors have found that by intentionally making uneven a distribution of the light intensity within the light beam width, crystallization can smoothly progress, and thereby a high quality crystalline thin film can be obtained. Based on this view, the following aspects of the present invention have been accomplished.
In accordance with another aspect of the invention, there is provided a method of producing a semiconductor device wherein a thin film of a non-single crystalline material formed on a substrate is irradiated with a light beam whereby the non-single crystalline material is crystallized or recrystallized to form a crystalline semiconductor thin film, the method characterized in that the light beam is such that a distribution pattern of a light energy intensity of the light beam is adjusted so that a temperature gradient or an uneveness of temperature distribution is caused, and the light beam is applied in a stationary state.
In the above-described method, uneveness of the temperature gradient or temperature distribution is caused on the surface of the non-single crystalline thin film irradiated with the light beam, and thereby it is made possible to prevent the phenomenon that micro crystal nuclei are simultaneously formed in a wide region, the phenomenon explained previously referring to FIGS. 7(f) and 7(g). Therefore, relatively large crystal grains are obtained, and evenness in the degree of crystallinity is increased. Consequently, the density of grain boundaries becomes small, and field effect mobility is improved.
In another embodiment of this aspect of the invention, a distribution pattern of the light energy intensity may be such a distribution pattern that a light intensity within a beam width monotonously increases from one side to the other, or monotonously decreases from one side to the other.
In this configuration, the temperature gradient on the surface of the non-single crystalline thin film to be irradiated is formed correspondingly to the light energy intensity, and the crystallization is guided in a direction from a region where the temperature is low towards a region where the temperature is high. Thus, disorderly formation of crystal nuclei and disorderly crystal growth are prevented, and consequently it is ensured that the phenomenon explained with FIGS. 7(f) and 7(g) is prevented.
In the case of employing the crystalline thin film for, for example, a semiconductor circuit comprising a source regionxe2x80x94a channel regionxe2x80x94and a drain region, it is preferable that the intensity gradient of the light energy be formed in the direction parallel to the source-drain direction. Thereby, the direction of crystal growth is restricted to the direction parallel to the direction of the transfer of carriers, and the density of grain boundaries becomes small. Accordingly, by employing this technique, a mobility of, for example, 300 cm2/Vs or higher can be achieved.
In another embodiment of this aspect of the invention, a distribution pattern of the light energy intensity may be such that, in a beam width, a part having a relatively stronger light intensity and a part having a relatively weaker light intensity are alternately arrayed in a plane.
When a light beam having a striped pattern made of a part having a strong light intensity and a part having a weak light intensity is applied, a striped temperature distribution pattern made of a part having a high temperature and a part having a low temperature is formed on the irradiated surface. In such a striped temperature distribution pattern, crystal growth is guided in the direction from a region where the temperature is low (normally formed in a belt-like shape) towards a region where the temperature is high. Then, crystal grains collide with each other in the vicinity of the center of the region (belt) where the temperature is high, forming a continuous line of grain boundaries (a continuous line like a mountain range) there, and crystal grains are formed so as to longitudinally extending in the direction parallel to the continuous line.
Hence, in this configuration as well, the phenomenon explained with FIGS. 7(f) and 7(g) is prevented, and moreover, the same advantageous effects as described in the above-described configuration employing such an intensity distribution monotonously increasing or decreasing are also obtained. Specifically, crystallization is effected while arranging a region with a relatively strong light intensity and a region with a relatively weak light intensity parallel to the source-drain direction. Thereby, the collision line of crystal grains becomes parallel to the source-drain direction, and it is prevented that carriers cross the collision line of crystal grains (the line of grain boundary), which causes a considerable decrease in the mobility. Thus, a channel region having a high mobility can be formed.
In the above-described method, a distribution pattern of the light energy intensity may be formed by causing a light interference by simultaneously irradiating with at least two coherent lights.
In this method utilizing a light interference, it is possible to form a fine light intensity distribution, and as a result to form a fine striped temperature distribution on the surface to be irradiated. Accordingly, this method achieves a smooth crystallization in a relatively wide region.
In another embodiment of the invention, a distribution pattern of the light energy intensity of may be a wave-motion-like interference pattern formed by simultaneously irradiating with at least two coherent lights and by dynamically modulating a phase of at lease one of the two coherent lights.
In this method utilizing a dynamic light interference pattern, the energy intensity distribution of the light beam varies in a wave-motion-like manner, and the temperature of the irradiated surface correspondingly varies in a wave-motion-like manner so that the temperature moves in one direction. Accordingly, by employing this method, impurities contained in the non-crystalline thin film can be gradually expelled outside the effective area, and consequently a crystalline thin film having a high purity and a high mobility can be formed.
It is noted that in the methods of producing a crystalline thin film according to this aspect of the invention, the light beam may be applied as the beam is being moved relative to the non-single crystalline thin film on the substrate. In this method in which a light beam is applied while being moved relative to the surface of the thin film, the light beam having a light energy intensity distribution pattern being adjusted so that a temperature gradient or an uneveness of the temperature distribution is caused on the surface to be irradiated (the surface of the non-single crystalline thin film), the orientation of crystal growth can be finely guided. Therefore, a high quality crystalline thin film having a high uniformity in the degree of crystallinity and a small density of grain boundaries can be obtained.
In accordance with another aspect of the invention, there is provided a method of producing a semiconductor thin film wherein a thin film comprising a non-single crystalline material formed on a substrate is irradiated with a light beam and thereafter cooled whereby the non-single crystalline material is crystallized or recrystallized, the method characterized in that a pressure of an atmosphere gas is maintained at more than a predetermined value to cause an uneven temperature distribution on a surface of the thin film irradiated with the light beam.
In this method, at a moment when the molecules of the gas constituting the atmosphere gas collide with the surface of the thin film and detach therefrom, the molecules deprive the thin film of heat, forming a low temperature region in a limited area. Thus, crystal nuclei are formed in the region, and the formed crystal nuclei facilitate the crystal growth, consequently preventing the phenomenon explained with FIGS. 7(f), and 7(g).
In the above-described method, the pressure of the atmosphere gas to be maintained at more than a predetermined value may be 10xe2x88x925 torr or higher where the atmosphere gas is a hydrogen gas.
When a laser annealing treatment is performed under a hydrogen gas pressure of 10xe2x88x925 torr or higher, the above-described advantageous effect is ensured by the movement of hydrogen molecules, which have a high specific heat.
In order to provide a solution to the foregoing and other problems, the invention also provides a method of producing a semiconductor film comprising the step of crystallizing a precursor semiconductor film, the step wherein the precursor semiconductor film formed on a substrate is irradiated with a first energy beam supplying the precursor semiconductor film with at least such an energy that the precursor semiconductor film can be crystallized, and with a second energy beam such that an absorption index of said precursor semiconductor film is smaller than an absorption index by said first energy beam and an energy supplied by said second energy beam is smaller than an energy capable of crystallizing said precursor semiconductor film.
According to this method, the second energy beam can easily reach a lower portion of the precursor semiconductor film and further the substrate. Thereby, the precursor semiconductor film is heated through the thickness direction and the substrate is also heated, reducing the temperature difference between the point while the first energy beam is being applied and the point after the irradiation with the beam is completed. Thus, the precursor semiconductor film heated and fused by being irradiated with the first energy beam is crystallized after the irradiation is completed, while being annealed. Therefore, the crystal growth is facilitated, and it is made possible to form relatively large crystal grains and reduce crystal defects, which improves electrical characteristics of the semiconductor film.
In the above-described method, the precursor semiconductor film may be an amorphous silicon thin film.
Thereby, a polycrystalline silicon thin film having good crystal quality and good electrical characteristics such as field effect mobility can be readily produced.
Further in the above-described method, the first energy beam may be such that an absorption coefficient of the precursor semiconductor film is approximately equal to or greater than the reciprocal of a film thickness of the precursor semiconductor film, and the second energy beam may be such that an absorption coefficient of the precursor semiconductor film is approximately equal to or less than the reciprocal of a film thickness of the precursor semiconductor film.
By employing this method, much of the first energy beam is absorbed in the vicinity of the surface of the precursor semiconductor film, whereas much of the second energy beam reaches the lower portion of the precursor semiconductor film and the substrate, and thus the precursor semiconductor film is efficiently heated as well as the substrate. Thus, after the irradiation with the first energy beam is completed, the precursor semiconductor film is annealed and the crystal growth is facilitated. Therefore, it is ensured that relatively large crystal grains are formed, and a semiconductor film having good crystal quality is formed.
Further in the above-described method, the first energy beam may be such that an absorption coefficient of the precursor semiconductor film is approximately 10 times or greater than the reciprocal of a film thickness of the precursor semiconductor film, and the second energy beam may be such that an absorption coefficient of the precursor semiconductor film is approximately the reciprocal of a film thickness of the precursor semiconductor film.
By employing this method, the precursor semiconductor film can be more efficiently heated, and a semiconductor film having further higher quality can be formed.
In the above-described method, the first and second energy beams have a different wavelength from each other.
By employing this method, the difference of the absorption coefficients as described above can be readily realized.
The foregoing energy beams having a different wavelength from each other may be, for example, such that the first energy beam is an energy beam having a single wavelength, and the second energy beam includes at least a wavelength component in a visible light range.
More specifically, the first energy beam and the second energy beam may be, for example, a laser light and an infrared lamp, a laser light and an incandescent light, or a laser light and an excimer lamp light.
In addition, as the foregoing lights having a different wavelength from each other, for example, the second energy beam may contain at least a wavelength component from a visible light range to an ultraviolet range, such as a xenon flash lamp light.
In addition, the first energy beam and the second energy beam may be a laser light.
When the laser beam is employed, the irradiation with the energy beam having a large energy density can be readily performed, and thereby it is made easy to efficiently heat the precursor semiconductor film and the substrate.
More specifically, for example, in the case where the precursor semiconductor film is an amorphous silicon thin film, the first energy beam may be one laser light selected from an argon fluoride excimer laser, a krypton fluoride excimer laser, a xenon chloride excimer laser, and a xenon fluoride excimer laser, and the second energy beam may be a laser light of an argon laser.
In addition, for example, in the case where the substrate is a glass substrate and the precursor semiconductor film is an amorphous silicon thin film, the first energy beam is one laser light selected from an argon fluoride excimer laser, a krypton fluoride excimer laser, a xenon chloride excimer laser, and a xenon fluoride excimer laser, and the second energy beam is a laser light of a carbon dioxide gas laser.
Each of the excimer lasers mentioned above is easy to obtain a large power and is easily absorbed in the vicinity of the surface of the amorphous silicon thin film. The laser light of an argon laser transmits through the amorphous silicon film to a certain degree and is easily absorbed throughout the thickness direction of the amorphous silicon thin film. The carbon dioxide gas laser transmits through the amorphous silicon thin film relatively well and is easily absorbed by the glass substrate. Hence, the amorphous silicon thin film can be efficiently heated, a polysilicon thin film having good crystal quality can be readily formed, and the productivity can be readily improved.
In the method according to this aspect of the invention, the first energy beam and the second energy beam may be applied to a belt-like shaped region in the precursor semiconductor film.
By applying the beams to the belt-like shaped region, heating can be performed with a uniform temperature distribution, and thereby it is made possible to easily form a semiconductor film having a uniform crystal quality and to reduce the time required for the process of crystallization.
In the method according to this aspect of the invention, a region in the precursor semiconductor film to be irradiated with the second energy beam may be larger than a region in the precursor semiconductor film to be irradiated with the first energy beam, and may include the region to be irradiated with the first energy beam.
In this method as well, heating can be performed with a uniform temperature distribution, and thereby it is made possible to easily form a semiconductor film having a uniform crystal quality.
In the method according to this aspect of the invention, the first energy beam and the second energy beam are incident approximately perpendicular to the precursor semiconductor film.
When each of the energy beams are incident approximately perpendicular to the precursor semiconductor film as described above, a variation in the irradiation with each energy beam is reduced, and thereby it is made possible to easily form a semiconductor film having a uniform crystal quality.
In the method according to this aspect of the invention, the second energy beam is applied at least prior to applying the first energy beam. Such applying of the second energy beam prior to applying the first energy beam may be performed by controlling the timings of applying of the energy beams, and further, may be performed in such a manner that the substrate on which the precursor semiconductor film is formed is moved, and the second energy beam is applied to a more forward position in the precursor semiconductor film with respect to a direction of moving of the substrate than a position where the first energy beam is applied.
By performing such applying of the energy beams, crystallization is effected by the first energy beam in a state where the semiconductor film and the substrate are sufficiently heated by the second energy beam, resulting in an efficient crystallization process.
In the method according to this aspect of the invention, the first energy beam may be intermittently applied, and the second energy beam may be continuously applied.
Specifically, the first energy beam may be a pulsed laser light, and the second energy beam may be a continuous-wave laser light or a lamp light.
By continuously applying the second energy beam as described above, it is made easy to heat the substrate and the precursor semiconductor film at a predetermined stable temperature, and in addition, by intermittently applying the first energy beam, heat conduction to the substrate is suppressed to prevent the fusion or distortion of the substrate caused by overheating the substrate. Thereby it is ensured that the crystallization of the precursor semiconductor film can be readily attained.
In the method according to this aspect of the invention, the first energy beam and the second energy beam may be synchronized with each other and intermittently applied. Specifically, as the timings of the irradiation, it is preferable that a time of irradiating with the first energy beam should be within a time of irradiating with the second energy beam, and should be two-thirds or shorter of an irradiation cycle of the second energy beam. For the energy beams, specifically, the first energy beam may be a pulsed laser light, and the second energy beam may be a pulsed laser light or an intermittently-operated lamp light.
By intermittently applying the first energy beam and the second energy beam as described above, it is easily made possible to irradiate a unit area with a large light energy, and therefore heating with a large energy can be performed while preventing the fusion and distortion of the substrate caused by overheating the substrate, which easily ensures the crystallization of the precursor semiconductor film. In particular, the pulsed laser can easily attain a large power and thereby readily heat a large area at a high temperature. Therefore, the time required for the step of crystallization can be easily reduced to improve productivity.
In the method according to this aspect of the invention, the first energy beam and the second energy beam may be applied so that the precursor semiconductor film is heated at a temperature of 300xc2x0 C. to 1200xc2x0 C., or more preferably at a temperature of 600xc2x0 C. to 1100xc2x0 C.
By heating the precursor semiconductor substrate at the temperature in the above-described range, the temperature variation in crystallization is made gentle and the crystal growth is facilitated while preventing crystal defects and uneven crystallization caused by the formation of micro-crystals in a partial region, and the formation of large crystal grains are readily made possible.
In addition, the method according to this aspect of the invention may further comprise a step of heating the substrate on which the precursor semiconductor film is formed with a heater. More specifically, for example, it is preferable that the substrate on which the precursor semiconductor film is formed should be heated at a temperature of 300xc2x0 C. to 600xc2x0 C.
By heating the substrate with the heater in addition to the second energy beam, the precursor semiconductor substrate, is more efficiently heated, and in addition, the crystal growth is readily facilitated by annealing. Moreover, in comparison with the conventional case of heating the substrate with the heater only, a predetermined heating temperature can be obtained within a shorter time, which easily achieves an improvement in productivity.
In the method according to this aspect of the invention, the first energy beam may be applied to a plurality of regions in the precursor semiconductor film, and the second energy beam may be applied to only a part of the plurality of regions.
By applying the second energy beam to only a partial region, crystallinity can be improved in a limited region, for example, which requires particularly high electrical characteristics, and therefore a necessary and sufficient crystallization can be effected by a crystallization process with a short time, which easily achieves an improvement in productivity.
In the method according to this aspect of the invention, the second energy beam may be such that an absorption index of the substrate is larger than an absorption index of the precursor semiconductor film. In addition, it is preferable that the first energy beam be such that an absorption coefficient of the precursor semiconductor film is approximately 10 times or greater than the reciprocal of a film thickness of the precursor semiconductor film.
More specifically, in the case where the substrate is a glass substrate, the precursor semiconductor film is an amorphous silicon thin film, the first energy beam may be one laser light selected from an argon fluoride excimer laser, a krypton fluoride excimer laser, a xenon chloride excimer laser, and a xenon fluoride excimer laser, and the second energy beam may be a laser light of a carbon dioxide gas laser.
By employing this method, much of the first energy beam is absorbed in the vicinity of the surface of the precursor semiconductor film, whereas much of the second energy beam is absorbed by the substrate, and thus the precursor semiconductor film is efficiently heated as well as the substrate. Thus, after the irradiation with the first energy beam is completed, the precursor semiconductor film is annealed and the crystal growth is facilitated. Therefore, it is ensured that relatively large crystal grains are formed, and a semiconductor film having good crystal quality is formed.
In accordance with another aspect of the invention, there is provided an apparatus for producing a semiconductor film by crystallizing a precursor semiconductor film formed on a substrate, comprising a first irradiating means emitting a first energy beam, and a second irradiating means emitting a second energy beam resulting in a smaller absorption index of said precursor semiconductor film than said first energy beam.
By employing the above-described apparatus, the second energy beam can easily reach a lower portion of the precursor semiconductor film and further the substrate. Thereby, the precursor semiconductor film is heated through the thickness direction and the substrate is also heated, reducing the temperature difference between the point while the first energy beam is being applied and the point after the irradiation with the beam is completed. Thus, the precursor semiconductor film heated and fused by being irradiated with the first energy beam is crystallized after the irradiation is completed, while being annealed. Therefore, the crystal growth is facilitated, and it is made possible to form relatively large crystal grains and reduce crystal defects, which improves electrical characteristics of the semiconductor film.
In the above-described apparatus according to this aspect of the invention, the second irradiating means is a lamp radially emitting the second energy beam, and the apparatus further comprises a concave reflector collecting the second energy beam.
By employing the above-described apparatus, efficient heating of the substrate and so forth is easily performed with a uniform temperature distribution, and thereby it is made possible to easily form a semiconductor film having a uniform crystal quality.
The above-described apparatus for producing a semiconductor film may further comprise a reflective plate in which one of the first energy beam and the second energy beam is reflected and the other one of the first energy beam and the second energy beam is allowed to transmit, the apparatus wherein the first energy beam and the second energy beam are incident perpendicular to the precursor semiconductor film.
When each of the energy beams are incident approximately perpendicular to the precursor semiconductor film as described above, a variation in the irradiation with each energy beam is reduced, and thereby it is made possible to easily form a semiconductor film having a uniform crystal quality.
In the above-described apparatus, specifically, in the case where the precursor semiconductor film is an amorphous silicon thin film, the first irradiating means may be one of an argon fluoride excimer laser, a krypton fluoride excimer laser, a xenon chloride excimer laser, and a xenon fluoride excimer laser, and the second irradiating means may be an argon laser.
In addition, in the case where the substrate is a glass substrate and the precursor semiconductor film is an amorphous silicon thin film, the first energy beam may be one laser light selected from an argon fluoride excimer laser, a krypton fluoride excimer laser, a xenon chloride excimer laser, and a xenon fluoride excimer laser, and the second energy beam may be a laser light of a carbon dioxide gas laser.
In order to provide a solution to the foregoing and other problems of prior art, the present invention also provides, in another aspect of the invention, a method of producing a semiconductor thin film comprising a step of irradiating a non-single crystal semiconductor thin film with an energy beam, said non-single crystal semiconductor thin film formed on a substrate having an image display region and a driving circuit region, said method characterized in that a first irradiation of said image display region is performed by using an energy beam having a line-like cross-sectional shape, and a second irradiation of said driving circuit region is performed at a higher energy density than said first irradiation by using an energy beam having a square-like cross-sectional shape.
The present invention also provides a method of producing a semiconductor thin film comprising a step of irradiating a non-single crystal semiconductor thin film with an energy beam, said non-single crystal semiconductor thin film formed on a substrate having an image display region and a driving circuit region, said method characterized in that a first irradiation of said image display region is a scanning irradiation such that said substrate is scanned by said energy beam in a relative manner and a region to be irradiated with said energy beam is shifted with a predetermined overlap, and a second irradiation of said driving circuit region is a stationary irradiation with a higher energy density than said first irradiation such that said energy beam is fixed with respect to said substrate in a relative manner.
Specifically, for example, in the thin film transistors constituting a liquid crystal display device, different laser irradiation methods are employed for a pixel region which requires a uniformity of semiconductor film characteristics and for a driving circuit region which requires characteristics (particularly high mobility). Specifically, when performing a laser annealing in which amorphous silicon is irradiated with a laser light so as to fuse and crystallize the amorphous silicon to form polycrystalline silicon, the energy density of the laser light applied to the driving circuit region in the substrate plane is made higher than the energy density of the laser light applied to the pixel region, so as to form polycrystalline silicons in the driving circuit region and in the pixel region each having different characteristics from each other. More specifically, for example, a first laser light irradiation is performed for the pixel region alone or for the entire surface of the substrate, and thereafter a second laser light irradiation is performed for the driving circuit region using a laser light having a higher energy density than the laser light used in the first laser light irradiation.
According to this method, the mobility of the polycrystalline silicon in the driving circuit region becomes higher than the mobility of the polycrystalline silicon in the pixel region, and the characteristics of the polycrystalline silicon in the pixel region can be made uniform in the plane.
In the above-described method, the first irradiation may be performed by using an energy beam having a line-like cross-sectional shape, and the second irradiation may be performed by using an energy beam having a square-like cross-sectional shape. Thereby, the laser annealing can be performed without rotating 90 degrees the stage for fixing the substrate.
Additionally, in the above-described method, the first irradiation may be a scanning irradiation such that a laser beam is applied a plurality of times while a position to be irradiated with the laser beam is being shifted, and the second irradiation may be a stationary irradiation such that the position to be irradiated is fixed. Thereby, the mobility of the polycrystalline silicon can be increased, and in addition, the uniformity is attained.
Further, the laser annealing may be performed in such a manner that a plurality of regions in the driving circuit region are irradiated with laser lights having different energy densities from each other, thereby forming polycrystalline silicons having different characteristics from each other. In this case, it is preferable that the laser annealing be performed so that a region in which a transfer gate in the latch or shift resistor and the other regions are respectively irradiated with laser lights having different energy densities.
Further, in the above-described laser annealing methods, it is preferable that the edge of the laser beam not fall on the TFT pattern.
In accordance with another aspect of the invention, there is provided an apparatus for producing a semiconductor thin film comprising an energy beam generating means, and means for homogenizing an energy beam emitted from the energy beam generating means by shaping the energy beam so as to have a predetermined cross-sectional beam shape and a homogeneous energy, the apparatus wherein a non-single crystal semiconductor thin film formed on a substrate is irradiated with the energy beam to effect crystal growth the apparatus characterized in that the apparatus further comprises a filter having a plurality of transmissivities different from each other, and the energy beam is applied through the filter to a plurality of regions in the non-single crystal semiconductor thin film in such a manner that each of the plurality of regions receives a different energy density from each other.
By employing this method, it is made possible to form a plurality of polycrystalline semiconductor film having different characteristics on a single substrate plane.
In the above-described method, the apparatus m ay be a laser annealing apparatus in which transmissivities of the mask are varied by an optical thin film so as to have the plurality of transmissivities different from each other, and this enables the distribution of transmissivities to accurately formed. In addition, the apparatus may be a laser annealing apparatus in which the mask and the window for applying the laser light to the substrate in the treatment chamber are integrated, and this enables the apparatus to have a simple construction and to reduce attenuation of the light energy.
In another embodiment of the invention, there is provided an apparatus for producing a semiconductor thin film comprising an energy beam generating means, and means for homogenizing an energy beam emitted from the energy beam generating means by shaping the energy beam so as to have a predetermined cross-sectional beam shape and a homogeneous energy, the apparatus wherein a non-single crystal semiconductor thin film formed on a substrate is irradiated with the energy beam to effect crystal growth, the apparatus characterized in that, the means for homogenizing is capable of selectively shaping the energy beam into a plurality of cross-sectional beam shapes.
By employing this method, a laser light with the most appropriate shape is applied to each position on the substrate.